1. Field of the Invention
This invention relates to sliding members such as piston rings used in internal combustion engines and methods for manufacturing the same.
2. Description of the Related Art
In recent years piston rings have been subject to harsh operating environments due to demands for increased engine output and reduced exhaust gas emissions. Many engines using present methods such as hard chromium plating and nitriding treatment for surface processing of sliding surfaces, are not durable enough to withstand this harsh environment. A hard film of materials such as TiN and CrN using a physical vapor deposition process has been proposed. A thick film of CrN in particular has been employed to meet demands for engines with increased durability.
The CrN film shows superior wear resistance and scuffing resistance. This film is easy to mass produce and is available in films exceeding 30 .mu.m. However since the CrN film is brittle, excessive repetitive stress on the sliding surfaces during operation generates cracks, which may cause peeling.
Accordingly, the applicant previously proposed a hard coating material having improved the sliding characteristics and toughness of CrN, and a sliding member covered with the hard coating material (see U.S. patent application Ser. No. 08/207,049, now U.S. Pat. No. 5,449,547).
This hard coating material is characterized in that 3 percent to 20 percent by weight of oxygen is contained in a solid solution state in a crystal structure of CrN and the Vickers hardness of the hard coating material is in the range of 1600 to 2200. The surfaces of the sliding member covered with the hard film composed of the hard coating material have improved scuffing resistance and toughness. Application of the hard coating material can be performed by an ion plating process.
However, since nitrogen gas and oxygen gas are used as process gases in ion plating, the base material surface of the sliding member becomes oxidized by the oxygen gas. As a result, the adhesion between the base material surface and the hard film may weaken.